Insulin-like growth factor-I (IGF-I) belongs to a family of polypeptides known as somatomedins. IGF-I is structurally and functionally similar to, but antigenically distinct from, insulin. In this regard, IGF-I is a single-chain polypeptide with three intrachain disulfide bridges and four domains known as the A, B, C, and D domains, respectively. The A and B domains are connected by the C domain and are homologous to the corresponding domains of proinsulin. The D domain, a carboxy terminal extension, is present in IGF-I but is absent from proinsulin. IGF-I has 70 amino acid residues and a molecular mass of approximately 7.5 kDa (see Rinderknecht, J Biol. Chem. (1978) 253:2769; and Rinderknecht, FEBS Lett. (1978) 89:283). For a review of IGF, see Humbel, Eur. J Biochem. (1990) 190:445–462.
IGF-I stimulates growth and division of a variety of cell types, particularly during development. See, e.g., EP 560,723 A and 436,469 B. Thus, processes such as skeletal growth and cell replication are affected by IGF-I levels.
Due to the widely varied clinical applications for IGF-I, compositions with desirable characteristics are in great demand and several IGF-I formulations have been made. See, e.g., U.S. Pat. No. 5,126,324. In particular, compositions with high concentrations of IGF-I are preferable for certain indications. Additionally, it is preferable to administer IGF-I compositions at physiological pHs. It is also preferable that the IGF-I in such compositions remain soluble and that the compositions are capable of storage for extended periods of time at refrigerated temperatures.
Physical parameters such as temperature and pH affect the solubility of IGF-I. For example, below about pH 5.0, IGF-I is soluble at concentrations of about 80–100 mg/ml while above pH 5.5, the solubility drops about ten-fold. Additionally, IGF-I is less soluble at lower temperatures. Thus, in order to provide IGF-I compositions capable of refrigerated storage, e.g., to retain stability, while still maintaining acceptable IGF-I solubility levels, compositions are now generally kept at a pH less than 5.0. Unfortunately, administration of IGF-I compositions at such nonphysiological pHs causes pain and irritation at the site of injection.
In order to overcome this problem, experimenters have attempted to formulate IGF-I in various buffers. For example, Fransson and Espander-Jansson, J Pharm. Pharmacol. (1996) 48:1012–1015, describe IGF-I compositions including 5 mg/ml IGF-I in isotonic saline, with phosphate buffer concentrations ranging from 5 to 50 mM, at pH 6.0–7.0. The authors found that pH 7.0 IGF-I preparations caused less pain than pH 6.0 preparations and that lower buffer strengths reduced pain at nonphysiological pHs. However, the authors concluded that IGF-I pH 7.0 preparations were not feasible due to the instability of IGF-I at this pH. International Publication No. WO 94/15584 describes isotonic IGF-I solutions at pH 5.5 to 6.5 with phosphate buffer present in an amount less than 50 mmol/L, which are reported to result in reduced pain upon injection.
Additionally, in order to avoid stability problems and prolong shelf life, protein formulations such as those including IGF-I are often provided in a freeze-dried form. See, e.g., U.S. Pat. No. 5,210,074, which describes dried IGF-I compositions that include a strong acid, such as hydrochloric acid, to increase the shelf life of the formulation. However, freeze-dried formulations require reconstitution prior to injection, which is inconvenient and can lead to dilution errors. Additionally, freeze drying is costly and time consuming. Thus, it would be advantageous to prepare IGF-I compositions with increased IGF-I solubility at pHs greater than pH 5.0 and at refrigerated temperatures.